Saturday, 22 August 2015

Isolated Theropod teeth from the Middle Jurassic of Niger.

Unlike Mammals, Dinosaurs
replaced their teeth throughout their lives, continuously growing new teeth and
shedding older, worn ones. This, combined with the hard enamel coating found on
all vertebrate teeth, makes isolated teeth the most commonly encountered
Dinosaur specimens, so being able to accurately assign teeth to specific
Dinosaur groups has great importance for understanding the origins and
distributions of such groups. Within the Theropods, some groups havevery distinctive teeth which allow
confident assignment of teeth by simple examination, for example Spinosaurid
teeth have deeply veined enamel and fluted ridges, as well as tiny denticles
(toothlets) on the carinae (cutting blades on the back of the tooth), however most
groups lack such obvious characteristics and therefore palaeontologists have
developed methods based upon morphometric analysis (mathematical comparison of
different measurements and angles on a tooth, bone or shell), which allow
isolated teeth to be compared to teeth found in situ on more complete fossil
specimens.

The first tooth, MUPE HB-142 is
also the best preserved, lacking only its tip, estimated to have been 31 mm in
height, laterally compressed (flattened sideways) and curved towards the tip.
It has an unserrated carina on its upper portion, and large, closely spaced
denticles, with a density of roughly 2.25 denticles per mm. The crown of the
tooth has undulations (wrinkles) close to the carina, and a slightly braded
enamel texture.

Morphometric analysis of MUPE
HB-142 suggests that it probably derived from a Megalosaurid, as it shows
affinities with Megalosaurus, Dubreuillosaurus and Afrovenator. However it also shows some
affinities with the basal Tetanuran Torvosaurus,
(the Tetanurans are a wider group which include the Megalosaurs, Allosaurs,
Spinosaurs and other groups, basal Tetanurans are Tetanurans that predate the
splitting of the Tetanurans into these groups) and falls within the morphospace
(complete range of size and shapes) of both Megalosaurus
and Allosaurus, though it is
closer to average values for Megalosaurus,
so the possibility that this tooth came from an Allosaurid cannot be ruled out.

The second tooth examined, MUPE
HB-118, is the largest tooth in the collection, with a total height of 37.47
mm. This is similar in shape to MUPE HB-142, with denticles on the top third
portion of the tooth and a similar braided texture to the enamel. The carina
has chisel-like denticles, with a density of 3 per mm.

This tooth also shows affinities
to the Megalosaurids Megalosaurus, Dubreuillosaurus and Afrovenator, as well as with the early Neotheropod Dilophosaurus (related to but not included in the early Tetanurans) and the basal Tetanuran Piatnitzkysaurus.
Like MUPE HB-142 it falls within the morphospace of both Megalosarus and Allosaurus,
but while MUPE HB-142 was closer to average values for Megalosaurus, MUPE HB-118 falls closer to average values for Allosaurus.However the similarity of MUPE HB-118 to the teeth of Dubreuillosaurus and Afrovenator, combined with its proximity
to another tooth assigned to a Magalosaurid and the absence of known Allosaurid
teeth from the Southern Hemisphere in the Middle Jurassic leads Serrano-Martínez
et al. to conclude that this is also
a Magalosaurid tooth.

The third tooth examined, MUPE
HB-125, is the smallest in the sample, and has a broken apex, and an estimated
crown height of 20.26 mm. It is laterally flattened with a curved tip, though
less so than MUPE HB-142 and MUPE HB-118. It has a carina occupying ¾ of the
length of the tooth, with convex denticles on its bottom part and chisel-like
denticles towards the top. The enamel of the tooth is unwrinkled, and shows
only weak texturing.

This tooth shows affinities to Allosaurus the Megalosaurids Dubreuillosaurus and Afrovenator,and the basal Tetanuran Erectopus,
but does not fall within the morphospace of any known Dinosaur group, lying
between the values of Allosaurids, Megalosaurids and Dromaeosaurids. Again the
close proximity of this tooth to other Megalosaurid teeth, and absence of known
Allosaurid teeth from the Southern Hemisphere in the Middle Jurassic leads Serrano-Martínez
et al. to conclude that this is more
likely to be a Megalosaurid than an Allosaurid tooth.

The final tooth examined, MUPE
HB-87, is less curved than the other specimens and 23.82 mm in height. It has a
deeply veined enamel texture, and week undulations on the labial (inner) side
of its carina. The carina itself has tightly packed chisel shaped denticles
towards its tip, with smaller more convex denticles lower down.

This tooth shows affinities to Allosaurus the basal Tetanuran Berberosaurus, but falls within the
morphospace of the Spinosaurid group. It shows the deeply veined enamel texture
typical of Spinosaurids (and never top date recorded in any non-Spinosaurid
Dinosaur), but lacks the fluted ridges also typical of the group. It shows
reduction in the size of denticles on the carina, but not to the extent seen in
later Spinosaurids, and has other features intermediate between Spinosaurids
and other Tetanuran groups, suggesting that it is an early member of the Spinosaur
group, probably close to the split between this group and the related
Megalosaurids.

The Spinosaurs are divided into
two groups, the Baryonychines from Laurasia (North America, Europe and Asia)
and the Spinosaurines in Gondwana (Africa, South America, India, Madagascar,
Antarctica, Australia and New Zealand, though the group is not known from all
these areas). However while the closely related Megalosaurs are well
established in the Middle Jurassic, almost all known Spinosaur specimens come
from the Cretaceous, making the origins of the group somewhat obscure. The Spinosaurines
are generally considered to be more derived than the Baryonychines, which led
palaeontologists to theorize that the group originated in Laurasia and spread
to Gondwana, however recent discoveries of possible Spinosaurid teeth from the
Middle Jurassic of Tanzania and Niger, combined with the presence of the group
in Australia, which split away from the rest of Gondwana early in the break-up
of the supercontinent, leads Serrano-Martínez et al. to suggest an alternative view, that the Spinosaurs
originated in Africa in the Middle Jurassic and from there spread to other
areas of Gondwana, as well as to Laurasia through Iberia and Europe.

Generalized palaeogeographic locations of spinosaurids (yellow) and the
specimen of HB site (red), through time from Bajocian/Bathonian, Tithonian,
Barremian−Aptian, and Albian−Cenomanian. Serrano-Martínez et al. (2015).

Terrestrial faunas of the Triassic comprised a wide variety of large Vertebrates, including Synapsids and both Dinosaurian and non-Dinosaurian Archosaurs. At the end of the Triassic a major extinction event wiped out or greatly reduced most of these groups, with the...

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About Me

Studied Palaeobiology & Evolution at the University of Portsmouth, Geosciences via the Open University & Ecology and Conservation at Christchurch University, Canterbury.
Have worked in wildlife based tourism, mineral exploration, development, conservation, education & environmental chemistry. Occasionally write articles for papers and magazines.

This Blog would be impossible without the work of countless scientists (and others) throughout the world. Where possible I do my best to credit them, but there will always be many more who remain unmentioned; this does not imply I am ungrateful for their contributions. Any errors or inaccuracies are, of course, my own.